Hackers siphoned off thousands of Healthcare.gov applications by breaking into the accounts of brokers and agents tasked with helping customers sign up for healthcare plans.

The Centers for Medicare and Medicaid Services (CMS) said in a post buried on its website that found that the hackers obtained “inappropriate access” to a number of broker and agent accounts, which “engaged in excessive searching” of the government’s healthcare marketplace systems.

CMS didn’t say how the attackers gained access to the accounts, but said it shut off the affected accounts “immediately.”

In a letter sent to affected customers this week (and buried on the Healthcare.gov website), CMS disclosed that sensitive personal data — including partial Social Security numbers, immigration status and some tax information — may have been taken.

According to the letter, the data included:

Name, date of birth, address, sex, and the last four digits of the Social Security number (SSN), if SSN was provided on the application;

Other information provided on the application, including expected income, tax filing status, family relationships, whether the applicant is a citizen or an immigrant, immigration document types and numbers, employer name, whether the applicant was pregnant, and whether the applicant already had health insurance;

Information provided by other federal agencies and data sources to confirm the information provided on the application, and whether the Marketplace asked the applicant for documents or explanations;

The results of the application, including whether the applicant was eligible to enroll in a qualified health plan (QHP), and if eligible, the tax credit amount; and

If the applicant enrolled, the name of the insurance plan, the premium, and dates of coverage.

But the government said that no bank account information — including credit card numbers, or diagnostic and treatment information was taken.

“Breaches that include personally identifiable information are always dangerous because they can lead to identity theft,” Andrew Blaich, Head of Device Intelligence at Lookout. “Not only can the attacker steal the identity of anyone in the breach, but they can also use this information to appear credible when crafting mobile spear-phishing messages against their targets.”

“This is especially true if the data that was leaked is accurate, as health information, family relationships and insurance information can make it extremely easy for an attacker to steal the identity of anyone affected by the breach,” he said.

President Obama’s healthcare law, the Affordable Care Act — known as “Obamacare” — allows Americans to obtain health insurance if they are not already covered. In order to sign up for healthcare plans, customers have to submit sensitive data. Some 11.8 million people signed up for coverage for 2018.

CMS previously said that the breach affected 75,000 individuals, but a person familiar with the investigation said that the number is expected to change. The stolen files also included data on children.

A spokesperson said CMS is expected to give an update early next week at the latest.